Comparing the energy performance of an electrochromic window under various control strategies
Introduction
Windows are considered an important factor for comfort in residential and commercial buildings. Natural light, fresh air and view are crucial for health and psychological reasons. Electrochromics were and are one the most promising technologies, in the sense that they represent active building elements. In comparison to opaque wall elements, windows have poor thermal characteristics, resulting in an increased energy consumption. Initial studies concentrated on the utility and economic benefits of electrochromic glazing [1], [2], [3]. Later studies [4] explored the potential advantages of hypothetical-properties electrochromics, since their dynamic change could meet the changing functional building requirements. As the first systems came to production, control strategies started to be investigated [5], while their impact on typical building energy consumption have been analysed in heating/cooling -dominated geographic locations [6].
Despite their energetic performance, electrochromics have been tested for visual quality against conventional glazing [7], [8]. To achieve this, a large dynamic range of transmittance is needed [12], a true challenge for the industry. The last 5 years research have been intensified on the development of control strategies in an effort to optimise the energy and comfort performance of these windows.[9], [10], [11]. These control strategies were based on (a) daylight (glare and/or illuminance) (b) incident solar radiation and (c) space cooling or heating consumption [13], [14].
A recent work for visual and energy management of an electrochromic glazing in Mediterranean climate was published in [15]. In this study the glazing controller was connected to the artificial lighting system. The heavy emphasis on the development of control strategies reflects the focus in most of the reviewed literature.
The principal goal of the present work was to develop an optimal decision algorithm in order to decrease the energy consumption while simultaneously improving the quality of the indoor environment. In particular, a fixed electrochromic glazing was placed on the southern wall of a PASSYS test cell and specific experiments have been carried out to optimise visual and thermal comfort as well as energy saving.
Section snippets
Experimental set-up
A series of experiments have been carried out in a PASSYS test cell [16] in an effort to measure various properties (transmittance, surface temperatures) of the electrochromic glazing and to develop a validated theoretical model for the forthcoming simulations [17]. The PASSYS test cell is a fully equipped, two zone, outdoor facility for thermal monitoring. The south façade of the cell is removable and allows installation and testing of specific building components. Experiments were carried out
Performance of the control strategies
A series of hour-by-hour building energy simulations were carried out for two periods of 90 days each during summer and winter, in order to test the relative performances of the developed control strategies.
The thermal and visual model of the PASSYS test cell was implemented in the SIBIL Building Toolbox environment [18]. The SIBIL Building Toolbox is a computational tool used for the thermal simulation of the buildings. Its realization takes place in a MATLAB-SIMULINK environment [19]. The
Results and discussion
Using the mathematical description of the above-mentioned controllers, simulation analysis has been carried out in order to compare their operation. Ranking was based on the criterion of minimum energy consumption. Simulation results concerning heating and cooling energy consumption are illustrated in Fig. 2. All numerical results are presented in Table 1, including heating, cooling and lighting consumption for all strategies. A closer examination leads to the conclusion that in terms of total
Acknowledgments
This paper is part of the work that was funded by the European Commission, Research Directorate General within the framework FP5 program Energy, Environment and Sustainable Development under the project SWIFT Switchable Façade Technology (ENK6-CT-1999-00012). The project is coordinated by the Fraunhofer Institute for Solar Energy Systems, Freiburg (Dr. Werner Platzer). Contact and project information can be found under http://www.eu-swift.de in the Internet.
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